This invention relates to an optical glass having a large negative anomalous dispersion value and having optical constants of refractive index (nd) within a range from 1.65 to 1.80 and Abbe number (.nu. d) within a range from 28 to 42 and moreover having no PbO and As.sub.2 O.sub.3 ingredients.
In an optical lens system, chromatic aberration can be removed for two color light by combining two types of normal optical glasses having different Abbe numbers, namely optical glasses having no large anomalous dispersion. As regards light of other colors, however, there still exists a residual chromatic aberration which is expressed as secondary spectra. Correction of such secondary spectra, particularly spectra in the region from blue spectrum of visible region to the ultraviolet spectrum, is desired in the design of optical lenses and such secondary spectra can be reduced to a substantial degree by combining an optical lens having a large positive anomalous dispersion with an optical lens having a large negative anomalous dispersion.
As optical glasses having optical constants within the above described ranges and having also a negative anomalous dispersion, known in the art are a SiO.sub.2 --B.sub.2 O.sub.3 --Al.sub.2 O.sub.3 --PbO glass disclosed in Japanese Patent Publication No. Sho-45-2311 and a SiO.sub.2 --B.sub.2 O.sub.3 --Al.sub.2 O.sub.3 --PbO--Sb.sub.3 O.sub.3 glass disclosed in Japanese Patent Application Laid-open No. Sho-48-74516. Since, however, these glasses contain a fairly large amount of PbO, they have a high density and a large weight and therefore have a disadvantage in using them. Moreover, these glasses do not have sufficient chemical durability and are costly for coping with environmental problems they have in manufacturing, processing and disusing them.
As glasses containing no PbO, Japanese Patent Publication No. Sho-51-34407 discloses a glass which contains SiO.sub.2, B.sub.2 O.sub.3 and GeO.sub.2 as glass-forming materials and also contains Li.sub.2 O, ZnO, ZrO.sub.2 and Ta.sub.2 O.sub.5. This glass, however, is economically disadvantageous because, for achieving desired refractive index and negative anomalous dispersion, it introduces a large amount of GeO and Ta.sub.2 O.sub.5 which are very expensive. Besides, this glass has difficulty in melting and therefore it is hard to obtain a homogeneous glass. Japanese Patent Publication No. Sho-58-46459 discloses a SiO.sub.2 --Nb.sub.2 O.sub.5 --R.sub.2 O--RO glass but this publication does not suggest anything about negative anomalous dispersion of the glass.
It is, therefore, an object of the invention to provide an optical glass having negative anomalous dispersion and having optical constants of refractive index (nd) within a range from 1.65 to 1.80 and Abbe number (.nu. d) within a range from 28 to 42, having excellent chemical durability and homogeneity and containing no PbO or As.sub.2 O.sub.3 which increases cost for protecting the environment and containing no expensive GeO.sub.2 or Ta.sub.2 O.sub.5 as an essential ingredient.